A rotor blade when moving in the same direction as the aircraft is called an advancing blade and when moving in the opposite direction is called a retreating blade. One of the primary factors limiting an operating envelope of a rotorcraft is onset of retreating blade stall. Retreating blade stall is a hazardous flight condition in helicopters and other rotary wing aircraft, where the rotor blade rotating away from the direction of flight stalls. Retreating blade stall is more likely to occur when the following conditions exist either alone or in combination: low relative airspeed and/or excessive angle of attack (or AOA), high gross weight, high airspeed, low rotor RPM, high density altitude, steep or abrupt turns, and turbulent ambient air.
Retreating blade stall is a limiting factor of a helicopter's airspeed, and the reason even the fastest helicopters can only fly slightly faster than 200 knots. In flight conditions at high speed, or even at slightly reduced speeds in the case of high aircraft gross-weight and/or altitude, retreating blade stall leads to a loss in lift, sharp increase in drag and pitching moment, globally reduces blade capabilities, and induces high vibration and control loads. Alleviating retreating blade stall is therefore of much interest when operating close to flight envelope boundaries, i.e. at high altitude, air speeds, or high gross weight.
In part because of the need for controlling retreating blade stall, and thereby expanding the operating envelope, there has recently been interest using active control and shape adaptation on helicopter rotor blades. Certain of these efforts have been directed to enhancing rotorcraft performance by reconfiguring a chord of the rotor blade. Examples of known modified chord configurations for rotor blades include Trailing-Edge Flaps (TEFs) and Gurney Flaps (GF).
A rotor blade having a trailing edge flap is depicted in FIG. 1A and literally includes a pivoting flap formed at a trailing edge of the rotor blade. The flap can be angularly adjusted relative to the remainder of the rotor blade air foil section, as shown.
A rotor blade having a Gurney flap is depicted in FIG. 1B and includes a small flat tab projecting from the trailing edge of a wing. Typically the Gurney flap is set at a right angle to the pressure side surface of the airfoil, and projects a distance of about 1% to 2% of the wing chord. The Gurney flap operates by increasing pressure on the pressure side, decreasing pressure on the suction side, and helping a boundary layer flow stay attached all the way to the trailing edge on the suction side of the airfoil.
While suitable to a certain extent, each of the trailing edge flap and Gurney flap are limited in their use. In particular, each of these devices maintains a non-adjustable fixed chord line dimension from a leading edge to a trailing edge of the chord.